A transmitter, such as a radio frequency transmitter used for communications, includes a power amplifier. It is known that a power supply to the power amplifier may be modulated to improve efficiency. In particular a peak voltage of the power supply is changed according to the peak voltage amplitude of the signal from the power amplifier, or an envelope tracking scheme is used to modulate the power supply voltage to the power amplifier according to the input to the power amplifier.
One problem with modulating or changing the power supply voltage to a power amplifier is that unwanted distortion may occur due to gain compression caused by excessive power supply voltage reduction. This may result in clipping the output signal from the power amplifier. Therefore, it is desirable to detect power amplifier compression and adjust the power supply voltage to the power amplifier accordingly. In one solution for detecting and adjusting for power amplifier compression, two signals are detected for use in determining power amplifier compression. These two signals are typically from two different points in the power amplifier. For example, the two signals are an input to the power amplifier and an output from the power amplifier, or a raw signal and a corrected signal that results by adding feedback to the raw signal. A compression detector senses the state of the two signals and generates a compression feedback signal. The feedback signal indicates whether there is deviation from an optimum compression level. Where there is undesirable deviation, a power supply modulator adjusts the supply voltage to the power amplifier. This algorithm for compression detection may be relatively computation intensive, which is not a problem for many applications. In some instances though, where compression detection is desired, there may not be access to two signals from different points in power amplification or there may be limited computation resources.
Therefore a need exists for improved compression detection for controlling a supply voltage to a power amplifier.